Method of coating paper with copolymers of vinyl alcohol and itaconic acid

ABSTRACT

The present invention is a composition useful for coating the surface of paper wherein the coating composition is an aqueous solution comprising less than 40 wt % water and consisting essentially of a polyvinyl alcohol copolymer that is at least about 95% hydrolyzed, and a process for preparing the same.

This application is a divisional of U.S. patent application Ser. No.11/012,516, filed Dec. 15, 2004, which claims priority from U.S.Provisional Application No. 60/529,855, filed Dec. 15, 2003, nowexpired, the entire contents being incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to polyvinyl alcohol copolymers for use in papercoatings.

2. Description of the Prior Art

In the process of producing coated paper, paper coatings are applied tothe surface of a paper product to obtain the coated paper. The papercoating can include various components, including polyvinyl alcohol.Other components may include calcium carbonate, clays such as hydrousaluminum silicates of the kaolin variety or hydrated silica clays,titanium dioxide, blanc fixe, lithopone, and zinc sulfide. Othercomponents of a conventional paper coating composition can include, forexample, mineral pigments, pigment binders, thickeners, humectants, andlubricants—all in various proportions. Paper coatings used to producecoated papers can include high percentages of solids, and such solidslevels can approach 70%. Polyvinyl alcohol can be used as a clear coatand also as a co-binder with latex in coated papers. Typically in papercoatings applications polyvinyl alcohol is used and applied as anaqueous composition that typically comprises other components. It canbe, and often is, desirable to use high solids paper coatings for moreefficient production of coated paper.

However, polyvinyl alcohol (PVA) is difficult to prepare as an aqueoussolution of more than 30% solids, and above this level dispersing thepolyvinyl alcohol in water becomes very difficult and the resultingsolution viscosity very high. To obtain even this degree of solution, anaqueous mixture that includes PVA is typically heated, or “cooked” toachieve complete solution and the resulting PVA solution is then addedto a paper coating composition. In paper coatings applications, theamount of water required to deliver an aqueous PVA solution isdetrimental, or at least undesirable, in the paper coating composition.It is much preferred for paper coatings applications to use as littlewater as possible and as a result mixtures having 70% or more of waterare not desirable in paper coatings applications. The difficulties indissolving polyvinyl alcohol in water can cause problems with theprocess of applying the coating. For example, there can be difficultiesgetting PVA dispersed into the coating mixture and evenly distributed onthe paper surface with insoluble PVA.

U.S. Pat. No. 5,527,852 describes a particulate PVA slurry as a papercoating agent which can be prepared using PVA having an average particlediameter of from 0.01 to 1 μm. The PVA slurry described therein is saidto be useful when applied to paper as a slurry wherein the PVA issubstantially insoluble in cold water, and thereafter dissolves duringthe drying process to form a film on the surface of the paper.

U.S. Pat. No. 5,057,570 describes a PVA that is partially hydrolyzed,which can be added without additional water to a paper coatingcomposition, and thereby avoids increasing the amount of water in thepaper coating composition. However, it can be desirable to use fullyhydrolyzed PVA rather than partially hydrolyzed PVA. Also, it can bedesirable to use a higher molecular weight PVA polymer than described inU.S. Pat. No. 5,057,570. Each of these differences can reduce thesolubility of the PVA in water, and thus negate the ability to dissolvethe dry PVA in a high solids paper coating composition. It is alsorecognized that if the partially hydrolyzed PVA is not given time tofully dissolve, quality problems with the coating or the coated papercan be encountered.

US 2004/0186034 describes a water-soluble PVA copolymer filmcomposition.

It can be desirable to dissolve a fully hydrolyzed PVA having arelatively high degree of polymerization in a high solids paper coatingcomposition.

SUMMARY OF THE INVENTION

In one aspect, the present invention is a composition useful for coatingthe surface of paper wherein the coating composition is an aqueouscomposition comprising less than 40 wt % water and consistingessentially of a polyvinyl alcohol copolymer that is at least about 95%hydrolyzed.

In another aspect, the present invention is a method for preparing ahigh-solids PVA-containing paper coating composition comprising the stepof: adding, as a dry solid component, a PVA copolymer comprising atleast about 1 mol % of itaconic acid as a comonomer and completelydissolving the PVA copolymer.

In another embodiment, the present invention is a method for preparing ahigh-solids PVA-containing paper coating composition comprising the stepof: adding, as a dry solid component, a PVA copolymer that is aterpolymer comprising less than about 8 mol % of itaconic acid as acomonomer.

In another aspect the present invention is a terpolymer compositioncomprising vinyl alcohol, methyl acrylate, and itaconic acid moieties,wherein: (i) the terpolymer comprises less than about 5 mol % of vinylacetate moieties; (ii) the methyl acrylate fragment is present in anamount of at least about 1 mol % up to about 9 mol %; (iii) the itaconicacid is present in an amount of at least about 1 mol % up to about 9 mol%, and, (iv) the methyl acrylate is in the form of a lactone.

DETAILED DESCRIPTION

In one embodiment, the present invention is a high solids paper coatingcomposition comprising fully hydrolyzed PVA. For the purposes of thepresent invention, “high solids” as the term is used herein shall denoteaqueous compositions which are prepared by dissolving or dispersingsolid materials in water, and where water is the only component that isa liquid at 25° C. and 1 atmosphere pressure. A high solids coatingcomposition of the present invention is one that comprises less thanabout 40 wt % water. It can be desirable in the practice of the presentinvention to use as little water as possible, and so any amount of waterwhich is less than about 40 wt % of the total coating composition isdesirable, with the proviso that the composition comprises fullyhydrolyzed PVA copolymer that is completely dissolved in the aqueousphase of the coating composition.

A coating composition of the present invention can comprise or consistessentially of other materials such as minerals and pigments, forexample, that are not completely dissolved and thus provide a coatingsuspension, slurry, or dispersion. For the purposes of the presentinvention no distinction will be made between a suspension, slurry, ordispersion and each will be considered as included in the term “coatingcomposition”.

A coating composition of the present invention can additionally compriseother components that are known and conventional for use in papercoating applications. For example pigments, minerals, fillers, pHcontrol agents, surfactants, latexes, or dyes—any of which can beconsidered conventional to paper coatings applications—are contemplatedas useful in the practice of the present invention.

A coating composition of the present invention can comprise PVA solidsup to about 20 wt % of the mineral portion of the coating. In oneembodiment, the PVA solids of the present invention is a PVA/acidcopolymer consisting essentially of the copolymer obtained bycopolymerization of vinyl acetate with itaconic acid, followed bycomplete hydrolysis of the acetate moieties to yield the PVA/acidcopolymer. For the purposes of the present invention, completehydrolysis shall mean hydrolysis of at least about 95% of the acetatemoieties. Preferably, the coating composition comprises from about 0.5%to about 20 wt % PVA solids relative to the weight of the pigment. Morepreferably, the coating composition comprises from about 1% to about 2wt % PVA solids.

PVA copolymers of the present invention can be obtained by known andconventional methods. PVA is typically obtained by polymerization ofvinyl acetate monomer, followed by conversion of the vinyl acetatepolymer to the vinyl alcohol polymer. PVA copolymers are obtained byintroducing one or more additional monomers into the polymerizationvessel with vinyl acetate, and hence the term is inclusive ofdipolymers, terpolymers, and/or higher degrees of copolymers.Terpolymers are obtained by copolymerization of three monomers.

As used herein, the term complete hydrolysis shall mean that asufficient amount of reagent has been added to convert at least about95%, preferably at least about 97%, and most preferably at least about99% of the acetate groups of a polyvinyl acetate copolymer. PVAcopolymers of the present invention may include other carboxylic acid orcarboxylic ester functional groups. In such copolymers, vinyl alcoholmoieties can react with acid or ester groups to form lactone functionalgroups. Therefore, in the practice of the present invention the extentof PVA hydrolysis is indicated by the presence of either or both vinylalcohol and/or lactone structures. A PVA copolymer of the presentinvention can comprise some residual un-hydrolyzed acetate moieties.Preferably a PVA copolymer of the present invention comprises less thanabout 5 mol % residual acetate moieties, more preferably less than about3 mol %, and most preferably less than about 1 mol %.

The PVA/acid copolymer of the present invention comprises from about 1mol % to about 10 mol % itaconic acid. Preferably the PVA acid copolymercomprises from about 2 to about 8 mol % itaconic acid, and mostpreferably from about 3 to about 6 mol % itaconic acid.

The degree of polymerization of the PVA acid copolymer is from about 400to about 4000. Preferably the degree of polymerization of the PVAcopolymer is from about 500 to about 2000.

In another embodiment, the PVA acid copolymer of the present inventionis a terpolymer comprising vinyl alcohol and itaconic acid components,and additionally comprising a second acid polymer component that iseither an acrylic or methyl acrylate unit. A PVA mixed acid terpolymerof the present invention can be preferred in some cases. For thepurposes of the present invention, functional equivalents of itaconicacid can include mono- and/or diesters of itaconic acid.

In another embodiment, the present invention is a method for preparinghigh solids aqueous paper coating compositions from a completelyhydrolyzed PVA acid copolymer. In the practice of the present invention,the completely hydrolyzed PVA acid copolymer can be added as a dry solidto water, or alternatively added in any manner that will provide anaqueous PVA paper coating solution comprising less than 40 wt % water.In the practice of the present invention, a heating step is not requiredfor effective dissolution of the PVA acid copolymer described herein.

The PVA paper coating solution composition can be prepared at atemperature of less than about 30° C. Particularly, the paper coatingcomposition can be prepared at a temperature above about 15° C. up toand including a temperature of about 30° C. Preferably, paper coatingcomposition can be prepared at a temperature of from about 18° C. toabout 30° C.

The method of the present invention can comprise various other steps orprocedures that are known and/or conventional in processes for preparinghigh solids paper coatings. Such steps as mixing or dispersing thecomponents are considered conventional.

In still another embodiment, the present invention is an articlecomprising a coated paper, wherein the paper comprises a coating of aPVA acid copolymer, wherein the copolymer comprises itaconic acid.

In still another embodiment, the present invention is a polyvinylalcohol terpolymer composition wherein the terpolymer comprises vinylalcohol, an alkyl acrylate, and itaconic acid moieties. The terpolymercomprises at least about 90 mol % vinyl alcohol. Preferably the vinylalcohol is present in an amount of at least about 91 mol %, morepreferably the vinyl alcohol is present in an amount of at least about92 mol %, and most preferably at least about 94 mol %. The terpolymerfurther comprises itaconic acid in an amount of from at least about 1.0mol % up to about 9 mol %, and an alkyl acrylate is present in an amountof from at least about 1 mol % up to about 9 mol %.

Equivalents of itaconic acid can be useful in the practice of thepresent invention. For example, 2-acrylamido 2-methylpropane sulfonicacid (AMPS) is a commercially available monomer that can be used in thesame manner as itaconic acid in the practice of the present invention.The preparation of PVA/AMPS copolymers is described in T. Moritani andJ. Yamauchi, Polymer, v39, p 553-557 (1998). Terpolymers of the presentinvention can be obtained by including alkyl acrylates in thecopolymerization reaction. The salt form of AMPS is preferably used inthe practice of the present invention, and preferred for use is thesodium salt (SAMPS), although any organic salt form would suffice foruse herein. AMPS salts such as SAMPS can be included in an amount offrom 1 to 7 mol %, preferably from 2 mol % to 6 mol %, most preferablyfrom 3 mol % to 5 mol %.

Alkyl acrylates suitable for use in the practice of the presentinvention include alkyl groups having from 1 to 10 carbon atoms. Suchalkyl groups can be selected from the group consisting of methyl, ethyl,propyl, isopropyl, butyl isomers including iso-butyl, n-butyl, andt-butyl, and isomers of pentyl, hexyl, heptyl octyl, nonyl and decylalkyl groups. Preferably the alkyl group has from 1 to 4 carbon atoms,more preferably from 1 to 3 carbon atoms, and most preferably the alkylgroup is methyl.

A terpolymer composition as described herein is soluble in a coldaqueous solution at a temperature of less than or equal to about 25° C.

Without being held to theory, the terpolymer comprises an alkyl acrylatethat essentially exists in the lactone form rather than in the acid orcarboxylate form. In a composition of the present invention, itaconicacid can exist in various forms, such as an acid, salt, lactone, or anycombination of these forms, depending on the chemical environment of theterpolymer. The capacity of the terpolymer to change form under variouschemical conditions is contemplated herein, and does not negate thefunction of the presently claimed invention as described and practicedherein.

EXAMPLES

The Examples are for illustrative purposes only, and are not intended tolimit the scope of the invention.

TABLE 1 Properties of Coated Paper Bright- Est. Coat ness² Weight¹ TAPPIWhiteness² Fluorescence³ Fluorescence Ex (g/m²) (points) CIE TAPPI(points) vs. Base Paper C1 none 86.9 82.9 2.0 0.0 C2 12 87.9 82.3 1.0−1.0 C3 12 89.8 89.1 3.0 1.0 C4 12 90.4 92.2 4.0 2.0 C5 12 90.5 92.6 4.12.1 C6 12 90.5 93.1 4.4 2.4 1 12 90.3 92.1 4.1 2.1 2 12 90.2 92.0 4.12.1 3 12 90.3 92.4 4.2 2.2 4 12 90.5 92.3 4.0 2.0 ¹Laboratory handdraw-downs done on pre-coated wood-free paper base stock 2807-20.²Testing done using Technidyne Brightimeter Micro S-5. ³Fluorescence wasmeasured according to TAPPI standard test method T-452.

TABLE 2 Example¹ C2² C3² C4² C5² C6² 1 2 3 4 Ultrafine Ground CaCO₃ 80.080.0 80.0 80.0 80.0 80.0 80.0 80.0 80.0 Omya Hydrocarb 90 Fine ParticleSize Kaolin Clay, 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 20.0 HuberHydragloss 90 SBR Latex, BASF Styronal 10.5 10.5 10.5 10.5 10.5 10.510.5 10.5 10.5 ND656 CMC, Noviant Finnfix 10, from 1.0 1.0 1.0 1.0 1.01.0 1.0 1.0 1.0 solution Polyvinyl Alcohol, from — — 1.0 1.5 2.0 — — — —solution Polyvinyl Alcohol, from dry 0.0 0.0 — — — 1.0 1.0 1.0 1.0 OBA,Blancophor P 0.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 pH target 8.5-9.08.5-9.0 8.5-9.0 8.5-9.0 8.5-9.0 8.5-9.0 8.5-9.0 8.5-9.0 8.5-9.0 ¹Theconcentration of latex, CMC, polyvinyl alcohol, and Blancophor ™ P OBAare based on 100 parts pigment. The target coating solids was 65%.Celvol ® 203S (Celanese) is a ground, partially hydrolyzed very lowviscosity polyvinyl alcohol. Celvol 103 (Celanese) is a fully hydrolyzedvery low viscosity polyvinyl alcohol. The Celvol materials were added assolutions to make sure the comparison was to fully dissolved Celvolmaterial. ²Not an example of the present invention.

These example numbers correspond to Table I

Comparative Example 1 (C1)

An uncoated base paper was tested for Brightness, Whiteness, HunterColor, and TAPPI Fluorescence, giving the results shown in Table 1.

Comparative Example 2 (C2)

A coating was made without any polyvinyl alcohol or fluorescent dyeapplied to the base sheet and tested.

Comparative Example 3 (C3)

The surface of an uncoated basepaper substrate was coated with 12 g/m²with the coating described in Table 2 with fluorescent dye and CMC butno polyvinyl alcohol promoter. The coated paper was tested forBrightness, Whiteness, Color, and Fluorescence, giving the results shownin Table 1.

Comparative Example 4 (C4)

The process of Example C3 was repeated except that in addition 1.0 partsper hundred of Celvol 203S solution was included in the mixture.

Comparative Example 5 (C5)

The process of Example C3 was repeated except that in addition 1.5 partsper hundred of Celvol 203S solution was included in the mixture.

Comparative Example 6 (C6)

The process of Example C3 was repeated except that in addition 2.0 partsper hundred of Celvol 103 solution was included in the mixture.

Example 1

The process of Example C3 was repeated except that in addition 1.0 partsper hundred of 6.4 mole % Itaconic acid copolymer, 60% neutralizationwas added dry to the mixture.

Example 2

The process of Example C3 was repeated except that in addition 1.0 partsper hundred of 4.2 mole % itaconic acid copolymer, 60% neutralizationwas included in the mixture as a dry component.

Example 3

The process of Example C3 was repeated except that in addition 1.0 partsper hundred of 1.8 mole % itaconic acid copolymer, 40% neutralizationwas added dry to the mixture.

Example 4

The process of Example C3 was repeated except that in addition 1.0 partsper hundred of 2.7 mole % itaconic acid, 2.9 mole % methyl acrylatecopolymer, 30% neutralization was added dry to the mixture.

Example 5

A 2 L polymer kettle was assembled with an overhead stirrer, a Claisenhead with a multi-inlet port fitting connected to 2 syringe pumps and atap—H2O condenser further attached to a N2 bubbler, an overheadthermowell with a thermocouple device, and a septum. The agitator was asingle PTFE paddle and the stir rate was set to 80 rpm. The kettle wascharged with 1000 g vinyl acetate (Aldrich Chemical co., Milwaukee,Wis., USA) 401 g MeOH (EM Science), 1.5 g itaconic acid (Aldrich), and0.6 g methyl acrylate (Aldrich) then degassed at rt for 20 minutes witha sparging tube. The kettle was then heated to reflux (ca. 65° C.) withan oil bath. Syringe pump #1 was charged with neat methyl acrylate.Syringe pump #2 was charged with a 24.7 wt % solution of itaconic acidin MeOH. 2 g of VAZO-64 (DuPont, Wilmington, Del., USA) dissolved in 100g of MeOH was added to the reactor. Syringe pump #1 was then activatedto feed at 0.0262 cc/min and syringe pump #2 was activated to feed at0.46 cc/min. The polymerization was refluxed for 159 minutes, then asolution of sodium nitrite (Aldrich) in 80 g of MeOH was added all atonce to halt the polymerization. The solids were determined to be 17.8%at this time. The polymer is thus 4.1 mol % itaconic acid and 1.6 mol %methyl acrylate. The polyvinyl acetate terpolymer was transferred to a 3L round bottom flask. The flask was attached to a rotary evaporator andMeOH/vinyl acetate azeotrope was removed at reduced pressure. Additionof 500 g of MeOH followed by azeotropic distillation was repeated 3times, at which time removal of vinyl acetate was judged substantiallycomplete. The polyvinyl acetate was dissolved in 700 g of MeOH andcharged to a 1 gallon explosion-proof stainless steel blender (EberbachCorp., Ann Arbor, Mich.). The blender was set to stir at 10,000 rpm then430 g of 25 wt % sodium methoxide in methanol (Aldrich) was addedthrough a small hole in the blender's cover. The ensuing heterogeneousmixture was stirred for 10 minutes, then 143 g of glacial acetic acid(EM Science) was slowly added. The mixture was stirred 2 minutes, thenfiltered. The polymer product was rinsed 3 times with MeOH and once withacetone, then dried in a vacuum oven at 80° C. overnight. 151 g of whiteterpolymer powder were isolated. The viscosity was determined to be 21.8mPa-s (cP) (4% solids aqueous solution at 20° C. (68° F.), determined byHoeppler falling ball method). Infrared analysis in ATR mode of thepolymer showed the presence of a lactone carbonyl peak at 1745 cm⁻¹, anacid carboxyl at 1705 cm⁻¹, and a sodium carboxylate at 1572 cm⁻¹.

Examples 6-10 were prepared in the same manner as described for Example5, except that the polymerization time was 175 minutes and the prechargeand feed rates of itaconic acid (IA) and methyl acrylate (MA) weremodified as shown in Table 3.

The itaconic acid copolymer of Example 11 was prepared by prechargingthe vessel with 1.41 g IA and 1.46 g MA then feeding a mixture of21.3:14.4:64.3 solution of IA:MA:MeOH at 0.393 cc/min for 163 minutes.

Itaconic acid copolymers of Examples 12-14 were prepared as described inExample 5 except that polymerizations were halted after 175, 134, and159 minutes, respectively. The copolymers of Examples 1 and 2 wereprepared on a 20 liter scale, feeding IA comonomer at a rate to afford6.4 and 4.2 mol % comonomer, respectively.

TABLE 3 MA MeOH Ex- IA Pre- Pre- Pre- IA MA am- charge charge chargePump 1 Pump 2 mol mol ple (g) (g) (g) (cc/min.) (cc/min.) % % 6 1.240.72 400 0.0314 0.368 2.3 1.4 7 1.51 0.83 400 0.0380 0.442 2.9 1.7 81.49 1.47 400 0.064 0.442 3.0 3.0 9 1.00 2.50 400 0.11 0.300 2.1 5.4 101.40 1.48 420 0.066 0.425 2.7 2.9 11 1.41 1.46 420 see text see text 2.02.1 12 1.41 0 400 0 0.41 2.4 0 13 2.43 0 400 0 0.26 1.8 0 14 2.41 0 4000 0.46 3.1 0

Example 15

Test of solubility of polymers. The cold water solubility of the resinswas measured by charging a 20 mL vial with 50 mg of polymer, a magneticstir-bar, and 3.0 mL of deionized water. The mixture was stirred at22-25° C. and the time to dissolve>95% of the polymer was measured. Theresults are tabulated in Table 4.

C7 is a 5-6 mol % methyl acrylate polyvinyl alcohol copolymer, >99%hydrolyzed, 17-23 cP viscosity.

C8 is a commercially available polyvinyl alcohol homopolymer, ca. 88%hydrolyzed, which is cold water soluble and is equivalent to Celvol203S.

C9 is a polyvinyl alcohol homopolymer>99% hydrolyzed with 27-33 cPviscosity.

Methyl acrylate copolymer or homopolymer are not cold water soluble.

TABLE 4 Polymer of IA MA Example mol % mol % Dissolution time  6 2.3 1.430 min.  7 2.9 1.7 30 min.  8 3.0 3.0 30 min.  9 2.1 5.4 17 min. 10 2.72.9 10 min. 11 2.0 2.1 24 min. 12 2.4 0 20 min. 13 1.8 0  5 days 14 3.10 20 min. C7 0 5-6 insoluble C8 0 0  4 min. C9 0 0 insoluble

1. A method of coating a paper article comprising the steps of: (1)preparing a high-solids paper coating composition having an aqueousphase, by adding to water, as a dry solid component, a copolymercomprising units of vinyl alcohol, alkyl acrylate, itaconic acid andoptionally vinyl acetate, said copolymer comprising i) less than about 5mol % vinyl acetate units; (ii) about 1 mol % to about 9 mol % itaconicacid units: (iii) at least about 90 mol % vinyl alcohol units and (iv)about 1 mol % to about 9 mol % alkyl acrylate units; thereby forming apaper coating composition wherein said copolymer is completely dissolvedin said aqueous phase of the paper coating composition and wherein theamount of water present in said aqueous phase is such that theconcentration of water in the paper coating composition is less than 40%by weight, based on the total weight of the paper coating compositionand (2) applying the paper coating composition to the surface of a paperarticle.
 2. A method of claim 1 wherein the alkyl acrylate units of thecopolymer are present in the form of lactones.
 3. A method of claim 1wherein said paper coating composition is a solution.
 4. A method ofclaim 1 wherein said paper coating composition is an aqueous mixture. 5.A method of claim 4 wherein said aqueous mixture is selected from thegroup consisting of aqueous slurries, aqueous dispersions and aqueoussuspensions.
 6. A method of claim 1 wherein the alkyl groups of saidalkyl acrylate units have from 1 to 10 carbon atoms.
 7. A method ofclaim 6 wherein the alkyl groups of the alkyl acrylate units have from 1to 3 carbon atoms.
 8. A method of claim 7 wherein the alkyl groups ofthe alkyl acrylate units have 1 carbon atom.
 9. A method of claim 1wherein the copolymer comprises less than about 3 mol % vinyl acetateunits.
 10. A method of claim 9 wherein the copolymer comprises less thanabout 1 mol % vinyl acetate units.
 11. A method of claim 1 wherein thepaper coating composition is prepared at a temperature of less than 30°C.
 12. A method of claim 1 wherein the copolymer is soluble in theaqueous phase at a temperature of less than or equal to 25° C.
 13. Amethod of claim 1 wherein the copolymer is at least about 95%hydrolyzed.